The aim of this program is to demonstrate the applicability of new synthetic principles and reactions to problems in organic chemistry. The new routes developed will lead to practical approaches to steroids, prostaglandins, and other natural products of biological significance as well as provide insight into the biosynthesis of such compounds. These goals center around the utilization of sulfur as a key element. A new annelation procedure is proposed which complements the Robinson process. The reaction involves secoalkylation followed by reclosure. Various alternatives are illustrated by synthetic approaches to ecdysones, ergosterol, vitamin D, and progesterone. Applications to other steroids important as anti-tumor agents in cancer chemotherapy and population control is obvious. Use of a novel photochemical cycloaddition and a sulfur based fragmentation, prostaglandins are easily accessible with the correct stereochemistry in both the ring and acyclic units. The role of sulfur in biogenesis of steroids, is also examined especially with respect to stereochemistry and utilization of the route in synthesis.